Functionalized anionically polymerized polymers of conjugated dienes and other monomers wherein the functionalization is terminal and/or internal are known. Particularly, U.S. Pat. No. 5,393,843 describes polybutadiene polymers having terminal functional groups. One of the methods described for making such polymers involves anionic polymerization utilizing a dilithium initiator such as the adduct derived from the reaction of m-diisopropenylbenzene with two equivalents of s-BuLi. Monomer is added to the initiator in hydrocarbon solution and anionic living polymer chains grow outwardly from the ends of the dilithium initiator. These polymers are then capped to form functional end groups as described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991. Of particular interest herein are terminal hydroxyl, carboxyl, sulfonate, and amine groups.
It has been observed that when the living polymer is reacted with the commonly available "capping" agents, the polymer in the hydrocarbon solution forms a gel. For purposes of this invention, a polymer gel is defined as a blend of a polymer and a hydrocarbon solvent that has a yield stress, that is, it will not flow unless it is acted on by at least some critical stress. A polymer gel as defined herein will require a significant application of force in order to initiate flow through an orifice. Of particular interest are gels that will not flow under the force of their own weight. The presence of gel that will not flow under the force of its own weight is readily detected by visual observation. This effect is observed by inverting a bottle containing the solution to see whether it flows to the bottom of the inverted flask. Gelled solutions will not readily flow to the bottom of the bottle.
The physical characteristics of these gels make them more difficult to handle in equipment which is designed for moving, mixing, or combining freely flowing liquids, i.e. materials without a significant yield stress. Pumps, reactors, heat exchangers, and other equipment that are normally used for making polymer solutions that can be characterized as viscous fluids are not typically suited to handling polymer gels. Thus, one would expect that processing equipment likely to be found at a manufacturing location that is designed to handle liquid polymer solutions, as defined above, would be ill suited to handling gels of this nature.
If the living carbon--alkali metal endgroups (chain ends) are first transformed to the "ate" complex (aluminate) by reaction with a trialkylaluminum compound, the addition of EO occurs nearly quantitatively, without the formation of gel. Addition of a trialkylaluminum compound can also dissipate a gel of this kind that has already formed. The molar ratio of the trialkyl aluminum compound to the polymer chain ends is generally at least 0.1:1, preferably 0.33:1 and most preferably 0.66:1 to 1:1 since this results in a freely flowing solution. Unfortunately, at the preferred aluminum levels, the hydrogenation activity of the Ni/Al catalysts that are often used in the hydrogenation of these polymers is poor. Substantially more catalyst and longer reaction time are required to reach an acceptable level of residual unsaturation in the trialkylaluminum--containing cements than in controls prepared in the absence of aluminum. The present invention provides a method whereby polymers using trialkylaluminum to mitigate the gel problem can be efficiently hydrogenated.
It is common practice to add an alkanol, such as methanol, to neutralize the basicity of the solution (known as the polymer cement) after the polymerization reaction prior to hydrogenation. Previously we found that addition of methanol at this point was preferable to omitting the alcohol or adding other alcohols, such as 2-ethylhexanol, but hydrogenation performance was poor compared to samples prepared without the added alkyl aluminum. We found that it was preferred to remove the aluminum and lithium by contact with aqueous mineral acid. This improvement resulted in a substantial improvement in both the rate of hydrogenation and the extent of hydrogenation at a given catalyst level. While this process represents a substantial improvement over the state of the art, it introduces an additional process step. It can be seen that it would be advantageous to accomplish the same result without the necessity of an additional process step.